Palladium membranes or palladium alloy membranes have selective permeability to hydrogen and deuterium, and are used as hydrogen separation membranes by virtue of this property.
When using the palladium membranes or the palladium alloy membranes as a hydrogen separation membrane, the smaller membrane thickness results in decrease in the hydrogen permeation rate and further decrease in the consumption of expensive noble metals such as palladium. Thus, a porous ceramic such as alumina is usually used as a support and a palladium thin membrane or a palladium alloy thin membrane is formed on the surface thereof by a plating method, such that the product is used as a hydrogen separation membrane (see Patent Document 1 described below).
On the porous ceramic, defects are usually present. When a palladium thin membrane or a palladium alloy thin membrane is formed by a conventional plating method, formation of defects (pinholes) on the membrane takes place more easily in case of the smaller membrane thickness. The pinholes cause reduction of the hydrogen purity (low hydrogen selectivity) after hydrogen separation and also cause degradation of membrane durability. As a method for preventing this, a method is disclosed wherein after carrying out a surface activating step of disposing a activating metal solution for electroless plating such that the pressure at one surface of a porous ceramic support is higher than the pressure at the other surface, to fill the activating metal solution in pores being open on the surface of the porous ceramic support, an electroless plating solution is disposed such that the pressure at one surface of the porous ceramic support contacting the electroless plating solution is higher than the pressure at the other surface to deposit a hydrogen separable metal such as palladium in the surface pores of the porous ceramic support, so that the pores are filled and choked with the hydrogen separable metal (see Patent Document 2 described below).
However, in this method there is a disadvantage that the hydrogen permeation rate decreases because the hydrogen separable metal is filled into the pores in the penetration depth of, for example, 30 μm (see Patent Document 3 described below). Thus, in Patent Document 3, after a surface activating step of disposing an activating metal solution for electroless plating such that the pressure at one surface of a porous ceramic support is equal to the pressure at the other surface to modify the surfaces of pores being open on the surface of the porous ceramic support with the activating metal solution, an electroless plating solution is disposed such that the pressure at one surface of the porous ceramic support contacting the electroless plating solution is higher than the pressure at the other surface to deposit a hydrogen separable metal such as palladium on the surface of the support and also in the surface pores of the porous ceramic support in a penetration depth of, for example, about 1 to 2 μm; thus, the hydrogen separable metal membrane is fabricated while choking tiny defects on the surface of the porous support. As a result, the hydrogen permeation rate is improved while the hydrogen selectivity is degraded. However, the metal in the pores still hinders hydrogen permeation, and a high hydrogen permeation rate is not always achieved with the hydrogen separation membrane.
The hydrogen permeance (k) of a hydrogen separation membrane whose main component is palladium generally follows the Sieverts' law. That is, k=J/(p10.5−p20.5), where J is a hydrogen permeation rate (mmol/s/m2), p1 is a hydrogen partial pressure (Pa) at inlet, and p2 is a hydrogen partial pressure (Pa) at outlet. For example, on the basis of the hydrogen permeation data at 500° C. for a palladium membrane with a thickness of 1 μm and a metal penetration depth into pores of 1.5 μm as described in Patent Document 3, the hydrogen permeance thereof is calculated as 1.9 mmol/s/m2/Pa0.5, and the hydrogen permeance at 500° C. for a palladium membrane with a thickness of 2 μm and a metal penetration depth into pores of 1.5 μm is 1.4 mmol/s/m2/Pa0.5. The hydrogen permeance of a palladium membrane generally follows Arrhenius' equation. That is, k=A×e−E/RT, where A is a frequency factor, E is an activation energy, R is a molar gas constant, and T is an absolute temperature. In general, the activation energy for a palladium membrane is about 10 kJ/mol, and the hydrogen permeance at 400° C. is estimated as 1.5 mmol/s/m2/Pa0.5 for a palladium membrane with a thickness of 1 μm from the above-mentioned hydrogen permeance at 500° C., and the value is 1.1 mmol/s/m2/Pa0.5 for a palladium membrane with a thickness of 2 μm.
For gases other than hydrogen, the gas permeance (k′) is generally expressed as k′=J′/(p3−p4), where J′ is a gas permeation rate (mmol/s/m2), p3 is a gas partial pressure (Pa) at inlet, and p4 is a gas partial pressure (Pa) at outlet.
A parameter of hydrogen selectivity, for example, is a ratio of the hydrogen permeation rate to the permeation rate of gases other than hydrogen at a pressure difference of 1 atm (hydrogen separation ratio, R). That is, R=J/J′=k×1013250.5/(k′×101325). For example, when 75% of hydrogen is separated from a raw material gas having a hydrogen concentration of 80 vol. % at an inlet pressure of 10 atm and at an outlet pressure of 1 atm, it is estimated that a hydrogen purity of 99.9% or above is obtained at R is 4000 or more.